1. Technical Field
The invention relates to apparatus for reading out the image stored in a photostimulable phosphor image recording medium. More particularly, this invention relates to apparatus for collecting and detecting the radiation emitted from the photostimulable phosphor in response to interrogation by stimulating radiation wherein flare is minimized.
2. Background Art
In a photostimulable phosphor imaging system, as described in U.S. Pat. No. Re. 31,847, reissued Mar. 12, 1985 to Luckey, a photostimulable phosphor sheet is exposed to an image-wise pattern of short wavelength radiation, such as X-ray radiation, to record a latent image pattern in the photostimulable phosphor sheet. The latent image is read out by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photostimulable phosphor releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. To produce a signal useful in electronic image processing, the photostimulable phosphor sheet is scanned in a raster pattern by a beam of light produced, for example, by a laser deflected by an oscillating or rotating scanning mirror. The emitted radiation from the stimulated phosphor is sensed by a photodetector, such as a photomultiplier tube, to produce electronic image signals.
In one type of scanning apparatus, the photostimulable phosphor sheet is placed on a translation stage and is translated in a page scan direction past a laser beam that is repeatedly deflected in a line scan direction to form a scanning raster.
To optimize the signal-to-noise ratio of the imaging system, it is desirable to collect as much of the emitted light as possible and to direct it to the photodetector. One form of light collector is proposed in U.S. Pat. No. 4,346,295, issued Aug. 24, 1982 to Tanaka et al. The light collector proposed by Tanaka includes a light guide member comprising a sheet of light transmitting material that is flat on one end and rolled into an annular shape on the opposite end. The flat end of the light collector is positioned adjacent to the scan line on the photostimulable phosphor sheet. The light receiving face of a photomultiplier tube is placed against the annular end of the light guiding member. Such a light collection system has the disadvantages of being expensive and inherently complicated to manufacture. Furthermore, the collection efficiency of transparent light guide members is limited due to their absorption in the wavelength range of light emitted by photostimulable phosphors.
In order to provide an easily manufacturable, low cost, high efficiency light collector, the present inventor proposed a double roof mirror light collector in U.S. Pat. No. 4,743,759, issued May 10, 1988, inventor John C. Boutet. As disclosed in this patent, a light collector for collecting and detecting light emitted from a photostimulable phosphor sheet in a photostimulable phosphor imaging system, includes a roof mirror light collector having a bottom roof mirror extending the width of the photostimulable phosphor sheet and a top roof mirror positioned over the bottom roof mirror to define a mirror box having a nearly square cross-section. The roof mirrors define slots along their peaks for passing a scanning beam stimulating radiation through the light box to the surface of the photostimulable phosphor sheet and for admitting emitted light from the photostimulable phosphor sheet into the light box. A photodetector is positioned at an end of the light box to convert collected light into an electronic signal representative of the latent image stored in the phosphor sheet. FIG. 13 of the above-mentioned patent, discloses a light collector which tapers from one end to the other with a light detector at the large end of the collector.
It has been found that stimulating radiation from the laser beam is reflected from the surface of the stimulable phosphor. If such stimulating radiation is rereflected to the stimulable phosphor, undesirable stimulation of the phosphor takes place which is called "flare". Flare causes undesirable image degradation and reduced image contrast which can affect proper diagnosis. Minimization of flare is therefore desireable.